Understanding multi-vent Plinian eruptions at Mt. Tongariro Volcanic Complex, New Zealand

Abstract

The andesitic Tongariro Volcanic Complex in the Central North Island of New Zealand comprises tens of vents along an actively extending graben. Past work has hypothesised that multiple vents may operate during a single explosive eruption episode, providing an unprecedented hazard challenge for this region. The stratigraphic record suggests an irregular clustering of eruption episodes over time, including an unusual ≤ 200-year-long period from ~11.3–11.1 ka when several large explosive eruptions occurred. We carried out new field mapping, stratigraphy, and sedimentology of the resulting pyroclastic deposits to identify seven episodes during this period, locating the vent sites and calculating their climactic dynamic properties. Five of the episodes involved two or more vent locations that produced separate sub-Plinian/Plinian eruption columns. Eruption columns ranged between 16 and 24 km high and erupted volumes of 0.05 to 0.76 km3, indicating magnitudes of 4 to 5. Each of these episodes show north-to-south “unzipping” of the Tongariro Graben, with episodes beginning in northerly vents, before being joined by vents south of these. This included two scenarios; northern edifice vents followed by central edifice ones or central edifice vents followed by southern ones. In both cases, the northerly vents generally display the greatest peak eruption magnitudes and the steadiest eruptions. By contrast, the southernmost vents formed along a fault-controlled fissure-like system. These produced unsteady plumes and pyroclastic density currents. The northward widening tectonic graben structure thus influences the pattern and style of these Plinian-style eruptions. This study highlights a real possibility for an extreme hazard scenario involving successive large-scale eruptions at volcanic systems in slowly rifting environments and sheds new light on the volcano-tectonic interaction in such settings.